Methods for the adsorptive purification of oils and fats have been known for a long time. Examples are for instance the extraction and purification of oils for the production of food, in particular by means of bleaching earth or activated carbon. These methods contact an adsorbent with the oil to be purified. The adsorbent, which is provided with many fine pores, takes up the pigmentations and dirt particles to be removed from the oil and, after the contacting process, is removed again from the oil, which is thereby purified and bleached.
The purification is frequently carried out discontinuously in a batch process or continuously in a uni-directional flow, i.e. oil and fresh adsorbent are mixed continuously or discontinuously and then separated again. In so doing a adsorbent separated still has free adsorption capacity.
It would be far more effective in theory to use the counterflow principle. This means that there is fed initially to the oil still to be purified not fresh adsorbent, but rather an already used adsorbent originating from a later purification stage. In relation to the oil still unpurified here, however, it still possesses sufficient potential for the take-up of dirt particles. After this first purification stage, consisting of contacting the absorbent with the oil and subsequent separation, the pre-purified oil is now treated in a further pass with fresh adsorbent, which is capable of taking up the smaller quantity of dirt and pigmentations particles still present and thereafter still possesses sufficient free capacity for a second application. The now used adsorbent can then, as indicated above, be added once again at a later point in time to the first contacting stage, while the oil now significantly purified by means of two stages may be further processed.
In principle it is also possible to provide more than two such separating stages.
Despite the theoretical advantage involved, the counter flow principle is not applied in practice because of the considerable cost of the equipment required. The benefits, namely the savings on adsorbent, bear no relationship to the requirements for an additional plurality of stage and guiding of the counterflow.
Attempts, using ingenious solutions, to make the counterflow principle more effective by means of continuous, simple solid or liquid phases flowing against one another are known for example from East German Patent Application 238 924. Rotating liquid columns and heavy and light phases rising or falling are used there. There has been no lack of attempts, in the case of columns with rotating baffles, to exploit a centrifugal field to improve the separation effect. In this case a stream of fluid is generated in a cylindrical tube, as a result of which heavy particles dispersed in the fluid extend at random to form into rings rotating about the main axis of the chamber. Said particles may be liquid particles or solid particles. A further alternative to this is known from Swiss Patent 382 716.
The disadvantage of centrifugal extraction consists in the high investment and operating costs. The counter-rotating transport of oil and bleaching earth in said column leads as a result of insufficient stage separation to a clear loss of propulsive force and reduces the maximum bleaching earth savings possible by virtue of the counterflow principle.
There is known from GB Patent 700 234 a method for the purification or bleaching of oils, in which an already substantially purified mixture of oil and adsorption agents is run through a press covered with filter cake. The filter cake contains already partially spent adsorption agents. The purified oil flows through the channels and pores of the filter cake, it being virtually impossible for any exchange of materials to take place and hence also a re-mixing of the contaminants from the filter cake with the purified oil being excluded from the outset.